1. Field of the Invention
This invention relates to a drive train of a vehicle, such as a mobile machine, with an internal combustion engine, a traction drive driven by the internal combustion engine, and working hydraulics driven by the internal combustion engine. The working hydraulics have at least one hydraulic pump driven by the internal combustion engine, which hydraulic pump can be operated as a pump and as a motor. When operated as a pump, the hydraulic pump takes in hydraulic fluid from a tank on a suction side (intake side) and delivers into a delivery line that leads to the working hydraulics. When the hydraulic pump is operated as a motor, the hydraulic pump is a hydraulic starter of a start-stop function to start the internal combustion engine, and is supplied with hydraulic fluid from a hydraulic accumulator on the suction side. A connecting line that leads to the hydraulic accumulator is connected to the delivery line of the hydraulic pump that leads to the working hydraulics.
2. Description of Related Art
Self-propelled mobile machines, such as industrial trucks, agricultural equipment, forestry equipment, and construction equipment such as excavators, wheeled and telescoping loaders, tractors, combine harvesters, forage harvesters, sugar beet and potato diggers, have a drive train with an internal combustion engine which drives a traction drive and which also drives the working hydraulics that perform the work functions of the machine. At least one hydraulic pump driven by the internal combustion engine is provided to supply the working hydraulics with hydraulic fluid.
During idle operation, when the traction drive and the working hydraulics are not actuated and, therefore, no torque is required from the internal combustion engine, the internal combustion engine is operated at a lower idle speed. Idle operation of this type occurs during pauses or interruptions in the work.
To reduce the fuel consumption of the internal combustion engine during pauses or interruptions in work, a start-stop function can be provided for the internal combustion engine, in which the internal combustion engine operating at no load is shut off during pauses or interruptions in work and is automatically restarted when there is a demand for torque from a work function or the traction drive. The shutoff and subsequent restarting of the internal combustion engine can occur even after relatively brief idle times, so that the starting process of the internal combustion engine is an operation that must be carried out correspondingly frequently and at short intervals during the operation of the internal combustion engine. This requirement places severe demands on the starter device of the internal combustion engine with regard to fatigue strength and the ability to deliver the starting energy required to start the internal combustion engine.
On internal combustion engines, such as diesel or gasoline engines, for example, starter devices driven by an electric motor are generally used. To start the internal combustion engine from a shutdown, the speed of rotation necessary for the self-sustaining running of the internal combustion engine is provided by an electric starter motor connected with the crankshaft of the internal combustion engine by a transmission. The transmission is generally formed by a pinion gear on the output shaft of the electric starter motor and a ring gear on the crankshaft of the internal combustion engine and has a high translation ratio so that a high-speed and compact electric starter motor can be used.
On known starter devices operated with electric motors with a high-speed and compact electric starter motor, very high currents are required to flow for a short period of time to produce the torque necessary to start the internal combustion engine. The currents that are generated to start the internal combustion engine result in a significant increase in temperature. If the internal combustion engine has to be restarted after short intervals for a start-stop function, this operating behavior results in adverse effects, such as the overheating of the electric motor, the failure of the electric starter motor, and, thus, of the starter device operated by the electric motor. To be able to actuate a conventional starter device of this type driven by an electric motor with an electrical starter motor for a start-stop function at brief intervals, the level of the electric voltage must be increased and the electric starter motor must be designed so that it has a correspondingly high fatigue strength, although that results in a significant increase in the design effort and manufacturing costs required.
On mobile machines, during the process of starting the internal combustion engine, the hydraulic pump of the working hydraulics, which is located in the drive train, also requires additional energy. Existing electric-motor driven, geared-down starter devices of the internal combustion engine are, therefore, unsuitable for economically creating a start-stop function which has the appropriate fatigue strength and supplies the necessary starting energy.
In drive trains of a similar type used in a mobile machine, hydrostatic power units are used as hydraulic starters of the internal combustion engine for a start-stop function. The hydrostatic power units are in a drive connection with the crankshaft of the internal combustion engine and are operated with hydraulic fluid from a hydraulic accumulator during a starting process of the internal combustion engine. DE 10 2011 105 006 A1 describes a drive train in which, in addition to the hydraulic pump of the working hydraulics, an additional hydraulic motor is provided that functions as a starter of the internal combustion engine to achieve a start-stop function of the internal combustion engine. Because an additional hydraulic motor is in coupled motion during the normal operation of the running internal combustion engine, losses occur which reduce the overall efficiency of the machine.
To eliminate the cost and effort required for an additional hydraulic motor as a starter of the internal combustion engine, the hydraulic pump that is already present for the working hydraulics can be used as a hydraulic starter of the internal combustion engine by operating the hydraulic pump as a motor supplied with hydraulic fluid from a hydraulic accumulator on the suction side. Drive trains of this type are known from FIGS. 1 and 2 of WO 2012/125798 A1. For charging of the hydraulic accumulator, in the system described by WO 2012/125798 A1, an electrically actuated charging valve in the form of a proportional valve that throttles the delivery line is located in the delivery line that leads from the hydraulic pump of the working hydraulics to the working hydraulics. Upstream of the charging valve, a connecting line that leads to the hydraulic accumulator and in which an electrically actuated shutoff valve is located is connected to the delivery line. The electrically actuated shutoff valve is actuated into an open position for charging of the hydraulic accumulator, and when a defined accumulator charge pressure (which is monitored by a pressure sensor) is reached, the shutoff valve is actuated back into the closed position to shut off the flow to the hydraulic accumulator. However, an electrically actuated shutoff valve of this type in the connecting line that connects the delivery line with the hydraulic accumulator results in high manufacturing costs and takes up a great deal of space. Additionally, a malfunction of the electrically actuated shutoff valve, such as a sticking in the open position, results in the charging of the hydraulic accumulator to unacceptably high accumulator charge pressures.
Therefore, it is an object of this invention to provide a drive train of the general type described above but which is improved in terms of manufacturing cost and space requirements for a charging circuit of the hydraulic accumulator and offers effective protection of the accumulator charge pressure.